Fluorescent lamps have been used to provide illumination for scanners. As an illumination source, fluorescent lamps provide the advantage of low operating temperatures. However, fluorescent lamps have several disadvantages. Typically, the visible light spectrum generated by a fluorescent lamp includes spikes (locations in the spectrum of emitted light for which the energy is above the desired nominal level) and holes (locations in the spectrum of emitted light for which the energy is below the desired nominal level). Furthermore, the spectral content of the light generated by fluorescent lamps shifts over time. In addition, the illumination intensity tends to vary over time. As a result, the scanner must frequently undergo calibration cycles to compensate for shifts in the spectral content of the lamp and the lamp intensity. Fluorescent lamps are generally the scanner component having the highest failure rate. As a result, replacement of the fluorescent lamp during the operating life of the scanner may be required.
Because the illumination characteristics of fluorescent lamps are temperature dependent, a warm up time in the range of one minute is generally required before use on both cold cathode fluorescent lights and hot cathode fluorescent lights. In some scanner applications, the time required from the initiation of scanning the first unit of media to completion is an important performance parameter. In order to reduce the time required for scanning the first unit of media, several techniques have been employed. In one technique, the fluorescent lamp is driven at higher than normal operating power in order to accelerate the warming of the lamp. In another technique, the lamp is continuously illuminated in order to eliminate the delay associated with the warming of the lamp. However, both of these techniques reduce the life of the fluorescent lamp, thereby worsening fluorescent lamp reliability problems. A need exists for an illumination source that improves upon these performance and reliability limitations.